57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

We have made significant progress towards our research goal of implementing helium nanodroplet isolation to trap and stabilize intermediates of prototype reactions that are central to combustion chemistry.  Intermediates that result from the barrierless reactions between hydrocarbon radicals or between a hydrocarbon radical and molecular oxygen are being characterized with infrared laser spectroscopy.  Furthermore, we have successfully trapped the hydroxyl radical (OH) in helium nanodroplets and probed the outcome of the OH + O₂ reaction, which sheds new light on the potential surface associated with this prototypical benchmark reaction. The vibrational frequencies, dipole moments, and vibrational transition moment angles obtained from these studies add new chemical insight into the detailed mechanisms of reactions that are relevant to low-temperature hydrocarbon oxidation and soot formation.  Furthermore, experiments of this type provide the mid-infrared spectral signatures of important combustion intermediates needed to develop laser diagnostic tools for modern chemical kinetics studies.

We reported the ro-vibrational spectrum of the nu₃(e') band of the methyl radical (CH₃) solvated in superfluid ⁴He nanodroplets.  Five allowed transitions produce population in the N_K = 0₀, 1₁, 1₀, 2₂ and 2₀ rotational levels.  The observed transitions exhibit variable Lorentzian line shapes, consistent with state specific homogeneous broadening effects.  Population relaxation of the 0₀ and 1₁ levels is only allowed through vibrationally inelastic decay channels, and the ^PP₁(1) and ^RR₀(0) transitions accessing these levels have 4.12(1) and 4.66(1) GHz full-width at half-maximum linewidths, respectively.  The linewidths of the ^PR₁(1) and ^RR₁(1) transitions are comparatively broader (8.6(1) and 57.0(6) GHz, respectively), consistent with rotational relaxation of the 2₀ and 2₂ levels within the vibrationally excited manifold.  The nuclear spin symmetry allowed rotational relaxation channel for the excited 1₀ level has an energy difference similar to those associated with the 2₀ and 2₂ levels.  However, the ^PQ₁(1) transition that accesses the 1₀ level is 2.3 and 15.1 times narrower than the ^PR₁(1) and ^RR₁(1) lines, respectively.  The relative linewidths of these transitions are rationalized in terms of the anisotropy in the He-CH₃ potential energy surface, which couples the molecule rotation to the collective modes of the droplet.

Helium nanodroplet isolation and infrared laser spectroscopy was used to investigate the CH₃ + O₂ reaction.  Helium nanodroplets are doped with methyl radicals that are generated from an effusive pyrolysis source.  Downstream from the introduction of CH₃, the droplets are doped with O₂ from a gas pick-up cell.  The CH₃ + O₂ reaction therefore occurs between sequentially picked-up and presumably cold CH₃ and O₂ reactants.  The reaction is known to lead barrierlessly to the methyl peroxy radical, CH₃OO.  The ~30 kcal/mol bond energy is dissipated by helium atom evaporation, and the infrared spectrum in the CH stretch region reveals a large abundance of droplets containing the cold, helium solvated CH₃OO radical.  The CH₃OO infrared spectrum was assigned on the basis of comparisons to high-level ab initio calculations and to the gas phase band origins and rotational constants.

The X²PI_3/2 hydroxyl (OH) radical has been isolated in superfluid ⁴He nanodroplets and probed with infrared laser depletion spectroscopy.  From an analysis of the Stark spectrum of the Q(3/2) transition, the lambda-doublet splittings are determined to be 0.198(3) cm^-1 and 0.369(2) cm^-1 in the ground and first excited vibrational states, respectively.  These splittings are 3.6 and 7.2 times larger than their respective gas phase values.  A factor of 1.6 increase in the Q(1/2) lambda-doublet splitting was previously reported for the helium solvated X²PI_1/2 NO radical [K. von Haeften, A. Metzelthin, S. Rudolph, V. Staemmler, and M. Havenith, Phys. Rev. Lett. 95, 215301 (2005)].  A simple model was developed that predicts the observed L-doublet splittings in helium solvated OH and NO.  The model assumes a small parity dependence of the rotor's effective moment of inertia and predicts a factor of 3.6 increase in the OH ground state (J=3/2) lambda-doubling when the B₀^e and B₀^f rotational constants differ by less than one percent.

The HOOO hydridotrioxygen radical and its deuterated analog (DOOO) have been isolated in helium nanodroplets following the in-situ association reaction between OH and O₂.  The infrared spectrum in the 3500-3700 cm^-1 region reveals bands that are assigned to the nu₁ (OH stretch) fundamental and nu₁+nu₆ (OH stretch plus torsion) combination band of the trans-HOOO isomer.  The helium droplet spectrum was assigned on the basis of a detailed comparison to the infrared spectrum of HOOO produced in the gas phase [E. L. Derro, T. D. Sechler, C. Murray, and M. I. Lester, J. Chem. Phys. 128, 244313 (2008)].  Despite the characteristic low temperature and rapid cooling of helium nanodroplets, there is no evidence for the formation of a weakly bound OH-O₂ van der Waals complex, which implies the absence of a kinetically significant barrier in the entrance channel of the reaction.  There is also no spectroscopic evidence for the formation of cis-HOOO, which is predicted by theory to be nearly isoenergetic to the trans isomer.  Under conditions that favor the introduction of multiple O₂ molecules to the droplets, bands associated with larger H/DOOO-(O₂)_n clusters are observed shifted ~1-10 cm^-1 to the red of the trans-H/DOOO nu₁ bands.

HO₃-(O₂)_n clusters are formed via the sequential addition of the hydroxyl radical and O₂ molecules to superfluid helium nanodroplets.  Infrared laser spectroscopy in the fundamental OH stretching region reveals the presence of several bands assigned to species as large as n=4.  Detailed ab initio calculations are carried out for multiple isomers of cis- and trans-HO₃-O₂, corresponding to either hydrogen or oxygen bonded van der Waals complexes.  Comparisons to theory suggest that the structure of the HO₃-O₂ complex formed in helium droplets is a hydrogen-bonded ⁴A' species consisting of a trans-HO₃ core.  The computed binding energy of the complex is approximately 240 cm^-1.  Despite the weak interaction between trans-HO₃ and O₂, non-additive red shifts of the OH stretch frequency are observed upon successive solvation by O₂ to form the larger clusters with n>1.